This invention relates to protective apparatus for three phase electrical devices. More particularly, this invention is concerned with electrical circuitry for protecting three phase motors against a loss of power on one or more input lines.
Multiple phase electrical devices, particularly three phase motors, are in widespread use today in industry. These devices are supplied by power from polyphase electric power sources which are subject to a variety of fluctuations in voltage and current because of power line failures, transients, ground faults, shorts and many other irregularities which affect the operation of the electrical devices and often cause damage to them.
In three phase electrical motors, it is estimated that approximately 90 percent of motor failures are due to conditions caused by power loss on some of the incoming lines from an electric power source. Typically, the power on one phase of a three phase system will fail, referred to as a single phase condition, resulting in excessive currents in the three phase motor and permanently damaging the motor circuitry.
A number of prior art devices have been developed for the purpose of detecting single phase power line failure and shutting down either the incoming polyphase source or the electrical apparatus attached to the source before damage can occur. These devices have generally been unsatisfactory for a number of reasons. Some sensing devices have not been flexible enough to accept substantial variations in voltage or current without disconnecting the system so that manual resetting is required before further power can be used. Moreover, typical prior art devices usually required either several mechanically moving parts or a substantial amount of electrical or electronic circuitry for detection and implementation of the sensing circuitry. Some systems involve the use of potentiometers and voltage meters requiring constant monitoring and adjustment to properly sense the incoming power lines.
Magnetic starters are frequently used in conjunction with three phase motors to enable automatic operation of the motors by remote control or by timing circuitry or other automatic means. Typically, a magnetic starter has one or more heater coils in series with its circuitry which emits substantial thermal energy when passing excessive current so as to open a thermal relay and shut off the motor. This type of heater coil has been found to provide sufficient protection in most cases against overloading of one or more phases of the circuitry. However, the coils provide little or no protection to the motor circuitry against the loss of power on a single incoming power line of a three phase power system. In most cases, the heater coils do not react quickly enough to prevent permanent damage to the motor circuitry before the motor can be turned off. Moreover, most magnetic starters use at most only two heater coils for a three phase motor so that at least one motor phase is especially vulnerable to a single phase condition.